It's worth quoting the O2 Ireland response, since it contrasts so much with Verizon's hardline attitude:

“When we see abnormal data use, we look into it and we find reasons that explain it, such as a customer buying a prepay smartphone with data capability, putting their old SIM into it without adding a data add-on to their account. These cases are exceptional and we deal with them as proactively as possible,” she says. Usage patterns are a good indicator of whether or not a customer is aware of what costs they are incurring on data. “If we see unusual activity we contact the customer directly . . . reach an agreement that is manageable.”